Couple questions involving gear ratios. How can gear ratio best be evaluated/measured/calculated? Is there a way to do a simple home calculation to come up with a ratio? How do cruisers of similar proportion (wheelsize/framesize) have completely different drive characteristics? Once measured, how can that ratio be recreated on another frame? Thx.
Gear ratio
- Thread starter nicninco
- Start date
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Calculating gear size in inches:
Crank teeth divided by cog teeth multiplied by wheel diameter.
Example:
52/11x27=127.64 gear inches
But you asked for ratio:
Wheel radius in mm / crank length in mm = X (The radius ratio)
X * front chainring teeth / rear cog = Ratio
Example:
A road bike with 170 mm cranks: (The usual generic diameter value for road wheels is 680 mm, so the radius would be 340 mm.)
340 mm / 170 mm = 2.0. (The radius ratio)
2.0 X 53 / 19 = 5.58
The frame is irrelevant.
Crank teeth divided by cog teeth multiplied by wheel diameter.
Example:
52/11x27=127.64 gear inches
But you asked for ratio:
Wheel radius in mm / crank length in mm = X (The radius ratio)
X * front chainring teeth / rear cog = Ratio
Example:
A road bike with 170 mm cranks: (The usual generic diameter value for road wheels is 680 mm, so the radius would be 340 mm.)
340 mm / 170 mm = 2.0. (The radius ratio)
2.0 X 53 / 19 = 5.58
The frame is irrelevant.
I'm the same way. Once it's together, if I can't get it up a hill, I put a smaller front sprocket, unless it's on a cruiser that has the front sprocket that it originally came with. Then I change the rear cog. Fortunately most old single speed cruisers have a comfortable setup. 48-52T front/16-20 rear, unless it's the skiptooths, then 26T/9-10T. My town is near the mountains and pretty hilly, but I can get up almost any of the hills on any of my old cruisers, so I'm content. I might start to sweat, but if you do it enough that will change. Plus I hate having too small of a sprocket up front, then having to pedal twice as much to make it across a flat intersection or stretch of busy road that I'd like to hurry up and get away from.
cruising on my mtbs (21 and 24 speeds) has taught me a few things about gearing bikes. i know its not quite the same, but its easier to shift and see how it feels than to swap out cogs and chainrings. actually, i need to leave the gears alone and lose weight and get into shape! 
:mrgreen:
:mrgreen:
The point of using some quick and easy calculations is that trial and error is not necessary. If there's a certain gearing that you like, it can be replicated or at least closely approximated across any crank-size/wheel-size combo with a cog/sprocket combination first shot.
MagicRat said:
Exactly. It's just math, and there are online calculators if you don't want to work it out yourself. I live on the side of a pretty steep grade, so I have to compromise my single-speed bikes, even if I ride them mostly on the bike path at the bottom of the hill. I've pretty much figured out what I can pull on the hill in terms of crank effort, and I figure out what chainring/cog combination I need to get there for a give tire size. It's just math.
